Silicon-based solar cells, due to an intrinsic optical band-gap of 1.1 eV, cannot absorb light having a wavelength longer than 1100 nm, e.g., infrared light. Light having a wavelength longer than 1100 nm does not have sufficient energy to excite an electron across the solar cell's band-gap, from a valence band to a conduction band, for photovoltaic energy conversion to occur. Because a significant part of the solar spectrum includes light having a wavelength longer than 1100 nm, silicon-based solar cells are currently unable to use approximately 30% of the sun's radiation for electricity generation. Persons having ordinary skill in the art to which the instant invention relates refer to this issue as the Shockley-Queisser limit of silicon-based solar cells.
Materials that convert low-energy photons, e.g., infrared light, into high-energy photons, e.g., visible light, are referred to as upconversion materials.
The highest efficiency upconversion materials currently known are based on rare earth ion co-doped NaYF4 particles which have quantum yields in the range of 0.005% to 0.3%. NaYF4 nanoparticles doped with 2% Er3+ and 20% Yb3+ are typical examples. The reported particle sizes for these rare earth doped NaYF4 particles ranges from 10 to 100 nm. A quantum yield of 3% has also been observed for a bulk sample of NaYF4 (Nanoscale, 2010, 2, 1417-1419).
Accordingly, there is a need in the related field for new materials and methods which overcome the Shockley-Queisser limit to silicon-based solar cells. The present invention provides new solid-state materials that upconvert infrared light into visible light, as well as methods of making and using the same, that are useful for overcoming the Shockley-Queisser limit and that have a surprisingly high infrared to visible light upconversion efficiency.